(1) Field of the Invention
The present invention relates to an organic light emitting element which consumes little power and has a long life.
(2) Related Art Statement
Organic light emitting elements are self-luminous, and therefore, make a backlight unnecessary when used in a display device, which is thus characterized by being thin and lightweight. Furthermore, organic light emitting elements have a wide view angle, and are characterized by their high response speed, and thus, are appropriate for moving image displaying devices. In addition, organic light emitting elements have a wide view angle and emit light through a surface, and thus, are appropriate for light emitting devices, such as for illumination.
In addition, organic light emitting elements have a structure where a lower electrode, a carrier transport layer, a light emitting layer and an upper electrode are layered on top of each other, and emit light as a result of recombination of electrons injected into the light emitting layer through an electron transport layer from the lower electrode, and holes injected into the light emitting layer through a hole transport layer from the upper electrode, for example.
Organic layers for organic light emitting elements and transparent electrodes for use as a contact electrode have a higher index of refraction than the air, which is a light extraction layer.
Therefore, it can be said that the intensity of light emitted into the air layer from the light emitting layer is approximately 20%, and most of the emitted light is totally reflected in the interface between the transparent electrode and the air layer and guided through the organic layer or transparent electrode layer.
Therefore, it is important to increase the efficiency of light emission, by reducing the loss when light is guided through the organic light emitting element, in order to implement an organic light emitting element which consumes little power.
In addition, it becomes possible to lower the current value required to gain a certain brightness by making the efficiency high in the organic light emitting element. It is known that the life of organic light emitting elements is proportional to the power of the inverse of the drive current, and thus, drive with a low current, as a result of the high efficiency, leads to a long life for the organic light emitting element.
Patent Document 1 discloses a structure provided with a light exuding diffusion layer in order to achieve an object of increasing the efficiency of light emission. A light exuding diffusion layer where a matrix having a low index of refraction contains transparent, microscopic particles for scattering light is provided on top of the second electrode, which is used as an electrode for emitting light.
Light which enters at an angle greater than the critical angle is totally reflected in the interface between the second electrode and the light exuding diffusion layer, and there is exuded light which propagates along the interface. Therefore, exuded light in the light exuding diffusion layer is scattered by the diffusing microscopic particles and propagates through the light exuding diffusion layer.
The results of our examination suggest that the structure disclosed in Patent Document 1 does not meet the conditions for increasing the efficiency of light emission. The details are given below.
In addition, Patent Document 2 describes a technology for increasing light emission by providing a light scattering layer where a first particle matrix and a second particle matrix contain transparent, microscopic particles.
In Patent Document 2, two types of matrixes with a different index of refraction contain transparent, microscopic particles, and the manufacturing method is advanced, making the cost high.    [Patent Document 1] Japanese Unexamined Patent Publication    [Patent Document 2] Japanese Unexamined Patent Publication 2005-353367